The thickening agents based on polymers or copolymers of unsaturated carboxylic acids, mainly acrylic acid and the copolymers thereof have been known for a long time. They are used, generally in neutralized form, in a variety of industrial fields, for example to thicken cosmetic and pharmaceutical formulations, paints, inks, detergents, pastes for textile printing and the like.
The commercial products are in the form of powders or liquid formulations or pastes consisting of the thickening agent, generally in the neutralized form, dispersed in a water/oil phase of hydrocarbon type.
The powdered products in the acid form have the advantage of providing the thickening agent in the pure state, and are suitable when no undesired substances are to be introduced. However they are difficult to use since they are exceedingly pulverulent and moreover a time-consuming dispersion thereof in a solvent (generally water) and a subsequent neutralization are necessary.
Powdered products in the neutralized form are already commercially available, but they suffer from a problem of being highly pulverulent.
The liquid or paste formulations, on the contrary, are easy to use in the already neutralized form. The use thereof is widespread in the textile field for the preparation of printing pastes not only since they are easy to use, but also above all because they provide high quality prints as far as colour yield, uniformity and contour sharpness are concerned.
However, the use of liquid or paste formulations necessarily involves the introduction in the printing pastes of high amounts of volatile organic compounds (V.O.C.), which are released in the air during the production. Therefore, the presence of V.O.C. involves problems of safety and environmental pollution.
For these reasons, the market of the thickening agents for textile printing tends to employ easy-to-use formulations with a reduced V.O.C. content or even free from V.O.C..
Liquid formulations of thickening agents free from V.O.C. consisting of emulsion polymers (latexes) are also on the market. However, said products have a very low viscosimetric yield, and therefore they have to be used in large amounts when a high viscosity is required.
According to this trend, Canadian Patent 2,085,481 discloses the preparation of solid thickening agents already neutralized and free from volatile powders, and of course from V.O.C.
Said products are acrylic acid polymers or copolymers, optionally cross-linked, obtained with techniques of inverse emulsion polymerization of acrylic acid salts and/or copolymers thereof. The preparation is carried out in the presence of polyalkylene glycols. The products are recovered by filtration after azeotropically removing the water from the system. The thickening agents obtained according to this process are substantially free from V.O.C. and dust-free. Their main use is the thickening of pastes for textile printing.
On the other hand, the compositions obtained according to this Patent involve some problems connected with the inverse emulsion preparation process. In particular, thickened gels of high transmittance, which is a paramount feature of many cosmetics in which the appearance is one of the more significant characteristics, cannot be prepared due to the presence of some synthesis additives which are water-insoluble, such as surfactants or the low HLB (hydrophilic-lipophilic balance) polymeric adjuvants necessary to prepare the inverse emulsion. Moreover, the resulting products are not very versatile, in fact they cannot be further formulated adding other ingredients conventionally used for textile printing pastes, to obtain ready-to-use formulations. As a matter of fact, the addition of other components during the polymerization step can negatively affect the process or the inverse emulsion destabilizing it, or make definitely more complex the process itself.